1. Field of the Invention
The present invention relates to the selective hydrogenation of contaminants contained in a light refinery stream.
2. Related Information
Dienes and nitriles are known catalyst poisons in processes using acid catalysts. Many important processes in the petroleum industry require acid catalysts. The production of gasoline octane enhancers such as methyl tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME) or catalyzed alkylation processes require acid catalysts.
Mixed refinery streams often contain a broad spectrum of olefinic compounds. This is especially true of products from either catalytic cracking or thermal cracking processes. These olefinic compounds comprise ethylene, acetylene, propylene, propadiene, methylacetylene, butenes, butadiene, etc. Many of these compounds are valuable, especially as feed stocks for chemical products. Ethylene, especially is recovered. Additionally, propylene and the butenes are valuable. However, the olefins having more than one double bond and the acetylenic compounds (having a triple bond) have lesser uses and are detrimental to many of the chemical processes in which the single double bond compounds are used, for example polymerization.
Refinery streams are usually separated by fractional distillation, and because they often contain compounds that are very close in boiling points, such separations are not precise. A C.sub.5 stream, for instance, may contain C.sub.4 's and up to C.sub.8 's. These components may be saturated (alkanes), unsaturated (mono-olefins), or poly-unsaturated (diolefins). Additionally, the components may be any or all of the various isomers of the individual compounds.
Several of the minor components (diolefins) in the feed will react slowly with oxygen during storage to produce "gum" and other undesirable materials. However, these components also react very rapidly in the TAME process to form a yellow, foul smelling gummy material. Thus it is seen to be desirable to remove these components whether the "light naphtha" cut is to be used only for gasoline blending by itself or as feed to a TAME process.
Diene contaminants can be removed by selective hydrogenation in the presence of olefins. The most recommended catalyst being palladium on a support, sometimes with promoters.
Hydrogenation is the reaction of hydrogen with a carbon-carbon multiple bond to "saturate" the compound. This reaction has long been known and is usually done at superatmospheric pressures and moderate temperatures using an excess of hydrogen over a metal catalyst. Among the metals known to catalyze the hydrogenation reaction are platinum, rhodium, cobalt, molybdenum, nickel, tungsten and palladium. Generally, commercial forms of catalyst use supported oxides of these metals. The oxide is reduced to the active form either prior to use with a reducing agent or during use by the hydrogen in the feed. These metals also catalyze other reactions, most notably dehydrogenation at elevated temperatures. Additionally they can promote the reaction of olefinic compounds with themselves or other olefins to produce dimers or oligomers as residence time is increased.
Selective hydrogenation of hydrocarbon compounds has been known for quite some time. Peterson, et al "The Selective Hydrogenation of Pyrolysis Gasoline" presented to the Petroleum Division of the American Chemical Society in September of 1962, discusses the selective hydrogenation of C.sub.4 and higher diolefins. Boitiaux, et al in "Newest Hydrogenation Catalyst", Hydrocarbon Processing, March 1985, presents an overview of various uses of hydrogenation catalysts, including selective hydrogenation, utilizing a proprietary bimetallic hydrogenation catalyst.
The known method of removal of nitriles from hydrocarbon feeds involves a water wash of the hydrocarbon feed. This requires a number of stages, depending on the relative solubility of the nitrile in water versus hydrocarbon. These additional stages provide additional production costs. It is particularly difficult to remove propionitrile by water wash. The additional stages required for water washing increase complexity and production costs. Although there is a substantial body of art relating to the hydrogenation of nitriles to produce amines or other amino compounds, there is no suggestion as to the fate of olefinic compounds during those processes.
U.S. Pat. No. 2,449,036 teaches the hydrogenation of nitriles to primary amines using nickel or cobalt catalysts providing the reduction is in the presence of a strong aqueous basic solution in ethyl alcohol.
U.S. Pat. No. 3,565,957 discloses the reaction of nitrilotriacetonitrile with hydrogen and a large amount of ammonia in the presence of a catalyst, chosen from a group consisting of nickel, cobalt, and rhodium.
U.S. Pat. No. 4,186,146 discloses the hydrogenation of aromatic nitriles to the corresponding aminomethylbenzene derivatives in a solvent system containing water, ammonia, and water miscible ether solvents using a cobalt or nickel catalyst.
U.S. Pat. No. 4,235,821 discloses the hydrogenation of aliphatic nitriles in a solvent system of water, ammonia, and water miscible ethers using a ruthenium catalyst.
U.S. Pat. No. 4,739,120 teaches the hydrogenation of an organic nitrile group to a primary aminomethyl group in the presence of a rhodium catalyst, a basic substance, and a two-phase solvent system comprising an immiscible organic solvent and water.
U.S. Pat. No. 5,075,506 describes a method for producing secondary amines from fatty nitriles, with ammonia and hydrogen over a cobalt catalyst promoted with zirconium. The catalyst may be supported on kieselguhr or other support. A second stage using the same catalyst but without ammonia, may be used to increase the proportion of secondary amines.
It is an advantage of the present hydrogenation process to selectively hydrogenate contaminants with little if any saturation of the olefins. The absence of hydrogenation of olefins is an unexpected benefit, since cobalt may used as a hydrogenation catalyst for olefins. A particular feature of the present process is that nitriles are hydrogenated to amines, which can be removed easily by water wash, when compared to nitriles, due to fact that the low molecular wt. amines are very soluble in water.